curl -X POST "{{BASE_URL}}/api/transactions" \
  -H "api-key: {{API_KEY}}" \
  -H "Content-Type: application/json" \
  -d '{
    "transactionType": "Sale",
    "idempotencyKey": "{{idempotencyKey}}",
    "cardData": {
      "cardNumber": "4111111111111111",
      "nameOnCard": "Jane Doe",
      "expirationMonth": 12,
      "expirationYear": 2030,
      "cvv": 123
    },
    "invoiceData": {
      "amounts": { "base": 10.00, "total": 10.00 }
    }
  }'

using var http = new HttpClient { BaseAddress = new Uri("{{BASE_URL}}") };
http.DefaultRequestHeaders.Add("api-key", "{{API_KEY}}");

// Chosen before the send, and kept, so a retry can reuse this exact value.
var idempotencyKey = "{{idempotencyKey}}";

var response = await http.PostAsJsonAsync("/api/transactions", new
{
    transactionType = "Sale",
    idempotencyKey,
    cardData = new
    {
        cardNumber = "4111111111111111",
        nameOnCard = "Jane Doe",
        expirationMonth = 12,
        expirationYear = 2030,
        cvv = 123
    },
    invoiceData = new
    {
        amounts = new { @base = 10.00m, total = 10.00m }
    }
});

response.EnsureSuccessStatusCode();

var sale = await response.Content.ReadFromJsonAsync<JsonElement>();
var transactionId = sale.GetProperty("id").GetString();
var merchantId = sale.GetProperty("merchantId").GetString();
var idempotencyStatus = sale.GetProperty("idempotencyStatus").GetString();

# -G moves the --data-urlencode values into the query string and encodes each
# one, and -X POST keeps the method. A key you chose may carry a character that
# means something in a URL, and an unencoded one asks about a different key.
curl -X POST -G "{{BASE_URL}}/api/transactions/get-by-idempotency-key-async" \
  --data-urlencode "merchantId={{merchantId}}" \
  --data-urlencode "idempotencyKey={{idempotencyKey}}" \
  -H "api-key: {{API_KEY}}"

var lookup = await http.PostAsync(
    $"/api/transactions/get-by-idempotency-key-async"
    + $"?merchantId={merchantId}&idempotencyKey={Uri.EscapeDataString(idempotencyKey)}",
    content: null);

if (lookup.StatusCode == HttpStatusCode.NotFound)
{
    // Nothing was charged under this key. This is the only answer that licenses
    // a resend.
}
else
{
    lookup.EnsureSuccessStatusCode();

    var existing = await lookup.Content.ReadFromJsonAsync<JsonElement>();
    var existingId = existing.GetProperty("id").GetString();
}

curl -X POST "{{BASE_URL}}/api/transactions" \
  -H "api-key: {{API_KEY}}" \
  -H "Content-Type: application/json" \
  -d '{
    "transactionType": "Sale",
    "idempotencyKey": "{{retryKey}}",
    "cardData": {
      "cardNumber": "4111111111111111",
      "nameOnCard": "Jane Doe",
      "expirationMonth": 12,
      "expirationYear": 2030,
      "cvv": 123
    },
    "customFields": [
      { "name": "loopback.latencyProfile", "value": "slow" }
    ],
    "invoiceData": {
      "amounts": { "base": 10.00, "total": 10.00 }
    }
  }'

var retryKey = "{{retryKey}}";

try
{
    var slow = await http.PostAsJsonAsync("/api/transactions", new
    {
        transactionType = "Sale",
        idempotencyKey = retryKey,
        cardData = new
        {
            cardNumber = "4111111111111111",
            nameOnCard = "Jane Doe",
            expirationMonth = 12,
            expirationYear = 2030,
            cvv = 123
        },
        customFields = new[]
        {
            new { name = "loopback.latencyProfile", value = "slow" }
        },
        invoiceData = new
        {
            amounts = new { @base = 10.00m, total = 10.00m }
        }
    });

    slow.EnsureSuccessStatusCode();
}
catch (TaskCanceledException)
{
    // Your client gave up. The request may still have completed, so the next step
    // is the probe, never a resend.
}

curl -X POST -G "{{BASE_URL}}/api/transactions/get-by-idempotency-key-async" \
  --data-urlencode "merchantId={{merchantId}}" \
  --data-urlencode "idempotencyKey={{retryKey}}" \
  -H "api-key: {{API_KEY}}"

// The recovery path, as your service would run it: the send threw, so ask.
var probe = await http.PostAsync(
    $"/api/transactions/get-by-idempotency-key-async"
    + $"?merchantId={merchantId}&idempotencyKey={Uri.EscapeDataString(retryKey)}",
    content: null);

if (probe.StatusCode == HttpStatusCode.NotFound)
{
    // Safe to resend, under the same key.
}
else
{
    probe.EnsureSuccessStatusCode();

    // It completed. Reconcile against this record and send nothing.
    var completed = await probe.Content.ReadFromJsonAsync<JsonElement>();
    var completedId = completed.GetProperty("id").GetString();
}